As set forth in U.S. Pat. No. 4,207,648 issued June 17, 1980, combustion of fuels in a utility boiler produces high amounts of particulate matter which accumulate on heated surfaces and reduces the heat transfer from the combustion to liquids to be vaporized. Coal firing produces large amounts of particulate matter, be it in the form of soot and/or slag. The lower the quality of coal, the more quickly is the accumulation of particulate matter on surfaces heated by the combustion. Removing structure must be frequently inserted into the furnace space to sheer away the accumulations which are the enemies of heat transfer.
Enter the lowly sootblower. Essentially, the sootblower is a tube, with a nozzle at its end, with means to insert it into a hole in the wall of the furnace. Steam, or other vapor, is fed into the tube and ejected from its nozzle with great force. Correctly positioned and directed, this vapor-belching tube can effectively sheer particulate matter from large areas of the heated surfaces.
In the huge, multi-storied utility boiler, it is not uncommon to mount up to 100 sootbloowers at external positions about the furnace. Horizontal rows of these blowers are poised at their furnace openings, the rows being vertically spaced from each other at approximately 8' intervals. Further, the nozzles are rolled into the furnace under elaborate programs to sequentially cut at the accumulations on the heating surfaces and maintain the efficiency of heat transfer from the combustion process to the vaproizable liquid behind the heating surfaces.
A vulnerable structure is the basic seal by which the cleaning vapor is retained within the outer casing of the lance. Although other vapors could be employed, by and large, high pressure steam is the most available cleaning medium. The steam is conducted to each blower though a feed tube. The outer casing of the lance, telescoped over the feed tube, is rotated and reciprocated over a substantial length of the feed tube, traveling into and out of the furnace space. Obviously, some form of seal between the outer surface of the feed tube and the inner surface of the outer casing of the lance is necessary to control the cleaning medium and force it from a nozzle mounted on the forward end of the casing. This basic seal must be protected from mechanical stress if reasonable life for it is to be expected.
The length of the outer casing of the lance, thrust into the furnace space, can be at least 50 feet. Fulcrumed near the entrance into the furnace, the nozzle-bearing end of the casing droops, or dipslaces vertically, a distance which depends upon the length of casing, the strength of the casing material, and the temperature to which the casing is subjected within the furance. Of course, this vertical displacement from the horizontal travel of the rear of the casing must be known in order to bring the nozzles into effective spatial relationship with the heating surfaces brought under the cleaning force of steam issuing from the nozzles.
It has been common practice to incline the lance toward the furance space so that condensing vapor within the outer casing and feed tube drains into the furance. The control of the droop of the nozzle end of the extended outer casing has been exerted by applying a downward force on the blower casing at a point horizontally spaced from the fulcrum at the furance entrance. This downward force has been predetermined to, in turn, establish the desired location of the extended nozzle end of the outer casing. However, the application of vertical forces to the structure, for bringing the nozzle to its desired position within the furnace, has distorted the feed tube and resulted in forces on its seal to the casing which has drastically shortened the life of the seal. Further, experience now indicates that the cooling effect of this condensed vapor dumped into the furance produces thermal shocks, and possible corrosion, on both the web welding between boiler tubes and tube studs which lead to tube failure.
The long-stroke sootblower lance must now be mounted to simplify the droop-correcting structure and obviate the dumping of the resulting condensed cleaning vapor toward and into the furance opening through which the lance casing reciprocates.